Keep a copy of the ifp in the uinet_config_if struct so some upcoming
commit stuff can reach back from uinet -> ifnet as appropriate.
Add two new interfaces types - bridge and span.

The bridge type is a simple 2-port passthrough bridge for doing
inline related interception.

The span type is a simple n-port aggregator interface that allows
for traffic to be coming in multiple interfaces but appear as a single
interface to the rest of the libuinet stack.
Merge pull request #18 from erikarn/adrian_iftypes

First version of bridge and span interfaces that solve a particular version of a particular problem. This is orthogonal to all else in libuinet, so there is plenty of time to review and consider how to generalize/improve for other applications.

The addition of the ifp pointer to uinet_config_if might be an indication that the config interface needs to be refactored.